Black holes are extremely efficient in transforming matter into energy, much more than nuclear reactions. In principle, there is a way to harvest the immense power generated by an accreting super-massive black hole, by surrounding it with an energy-collecting megastructure. Fabio Pacucci describes this very far-fetched idea which could enable the expansion of humanity in our Galaxy.

The outstanding efficiency of a “black hole engine” in transforming matter into energy is the main reason which led, in the 1960s, to the identification of the first super-massive black hole. In this Dig Deeper section, I will describe the enlightening (!) story of 3C 273, the first super-massive black hole ever identified.

The object 3C 273, nowadays known as the “prototypical quasar”, was included in the 1959 version of the Third Cambridge (3C) catalog of Radio Sources, and numbered as its 273rd source. This was considered a rather boring source until, in 1963, the astronomer Maarten Schmidt published a paper in Nature noting that 3C 273 was extremely far. With a redshift of z=0.158, it was about 2.5 billion light-years away. This source looks like an extremely luminous point, or, as astronomers say, it is point-like, just like a faraway star. The origin of the word quasar, used to designate far and accreting super-massive black holes, is indeed “quasi-stellar radio source”, because it emits radio waves and it is point-like.

The peculiarities of 3C 273 are multiple, but the most relevant ones are the following:1) This source is extremely bright, several trillion times brighter than our Sun.2) This source is extremely far, about 2.5 billion light-years away.3) This source varies significantly on very short timescales.

Because the source is extremely bright and very far, it follows that the energy released by whatever mechanism is at play in 3C 273 is huge. In addition, the short-term variability led to very important constraints on the physical size of the source of radiation, which cannot be larger than about 1 parsec, or approximately the distance to the closest star, Proxima Centauri. Although the physical size of the source may not seem to be so important at first thought, it is actually the crucial clue to understand the true nature of 3C 273. The immense energy radiated by 3C 273 cannot come from stars (or, in other words, from nuclear fusion) as there is no way to pack some trillions of stars inside a sphere of about 1 parsec. For comparison, a sphere of 1 parsec in radius centered around our Sun contains exactly one star (the Sun!).

After excluding other sources of radiation, scientists concluded that only one engine is efficient enough to produce this immense energy in a very compact region: the gravitational engine of a super-massive black hole, swallowing large quantities of materials into its endless pit. It took the work of several outstanding scientists, including Edwin Salpeter and Yakov Zeldovich in the 1960s, to finally understand the real nature of these mysterious sources.

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